For the most part, steel making is a batch procedure involving several steps. Hot molten metal (iron-carbon alloy with impurities) is produced continuously in a blast furnace to produce iron. The molten iron is transported batch-wise, optionally with some scrap steel, to a primary converter, for example a basic oxygen furnace, and is transformed into steel (primary steelmaking) by blowing oxygen to remove carbon and phosphorus. Alternatively, scrap steel may be melted, for example, in a primary electric arc furnace, with oxygen blowing to remove carbon and phosphorus. In both of the above primary steelmaking operations, the steel is usually formed in the presence of a primary slag, mostly composed of oxides including calcium, silicon, iron, manganese, phosphorus, chromium and aluminum. The primary slag is "oxidizing" with respect to acceptable steel oxygen contents prior to casting, and therefore it is not suitable for further steel refining operations. In the past, primary slag was modified within the primary steelmaking vessel and a secondary refining step was carried out to adjust the steel and primary slag compositions and temperature. More recently, such refining steps have been carried out outside of the primary steelmaking vessel, usually in a transfer ladle used to transfer molten steel to a pouring or casting location.
The term ladle, as used herein, describes a vessel usually having a consumable refractory lining which is used for transferring molten metal, particularly steel, from one place to another, for example, from a primary steelmaking furnace to a continuous caster.
Steel refining in the ladle has become common practice in recent years, and is now often combined with arc reheating in the ladle to maintain and control temperature. The ladle slag (secondary ladle slag) is an important aspect of ladle refining because its chemical and physical properties influence the economics of production and the quality of the final steel product. In most ladle refining practices, the molten steel is tapped from the primary steelmaking vessel into a ladle substantially free of primary slag. Alternatively, primary slag may be substantially removed subsequent to tapping into a ladle by raking or similar known processes. Additions are then made to the substantially slag free steel in the ladle to synthesize a new slag with desirable properties, usually referred to as a synthetic ladle slag or secondary ladle slag. Another approach to eliminating primary slag is tapping the primary slag along with the steel, and then treating the primary slag so that it becomes suitable for secondary refining.
Secondary ladle slags should provide various combinations of the following functions and characteristics adapted to specific applications:
7. Protect the steel from contact with the atmosphere, and;
Individually, components of the secondary ladle slag are not molten at steel refining temperatures. However, by ensuring such components are present in the proper proportions, in the proper particle size ranges and well mixed, it is possible to achieve a molten slag at steel refining temperatures through a process of dissolution of components into each other. To expedite the dissolution process, ladle slag additives should be well mixed together and should be selected so that the individual particles are sized small enough to promote quick dissolution and large enough to ensure even spreading of the added material in the ladle slag across the melt surface. Conventional practice utilizes particle sizes preferably in the range of about 0.50 to about 1.50 inches although small particle sizes may occasionally be employed for specialized purposes. Although this is known in the art, there is disagreement as to the appropriate size of particles. The desirability of thorough mixing of the components prior to addition to the ladle is not widely recognized.
In practice, the dissolution of ladle slag additives to form a continuous molten oxide phase is rarely achieved without the use of fluidizers such as calcium fluoride. A disadvantage of fluidizers such as calcium fluoride is that they can dissolve ladle refractory linings.
Slag additive dissolution may also be expedited by arc heating in the ladle. The intense heat from an arc may cause the slag additive components to melt and dissolve more rapidly. However, overheating of the slag may also increase solubility of the refractory lining material. Another problem with arc heating is that refractory lining wear can be increased by arc instability which can cause hot gas and slag to be propelled against the ladle refractory lining.
Reducing arc instability by using a fluid ladle slag of high basicity is known in the art, as described by Oliver et al., "International Symposium on Ladle Steelmaking and Furnaces," Metallurgical Society of CIM, Aug. 28-31, 1988, pages 130-143. Stabilizing the arc by injection of argon through an axial hole in the electrode is also known in the art, as described by Segsworth, U.S. Pat. No. 4,037,043, and by Oliver et al., "Plasma Heating for Ladle Treatment Stations," Iron and Steelmaker, July 1989, pages 17-22. These measures can reduce arc instability and thereby reduce the wear rate of refractory linings but they do not eliminate arc instability and it is desirable to find additional ways to promote better arc stability.
Another known approach to stabilizing arcs is the foaming of a slag to engulf the arc, for example, as described in U.S. Pat. No. 4,447,265 and in U.S. Pat. No. 4,528,035. These procedures involve the injection of carbonaceous material, lime, and oxygen into a primary steelmaking furnace during the refining stage. This process is said to be effective in increasing slag volume and protecting the furnace refractory walls from excessive wear. The injection of oxygen, however, produces an oxidizing atmosphere, which is not appropriate for use in ladle arc refining where reducing conditions are desirable. This is unfortunate, since the refractory lining lifetimes could be increased by such a process.
In U.S. Pat. No. 4,198,229, Katayama et al. discuss the use of calcium carbide as a component of synthetic slag for dephosphorization of alloy steel, stainless steel, or ferrochromium. In this technique a combination of calcium carbide, alkali metal halide, and, optionally, calcium metal alloys creates a condition whereby metallic calcium becomes available within the slag phase at the slag/metal interface. This metallic calcium can then combine with phosphorus dissolved in the metal phase to form Ca.sub.3 P.sub.2 which is assimilated into the slag phase, thereby dephosphorizing the metal. The purpose of the calcium carbide is to dephosphorize not to deoxidize the metal or the slag.
In U.S. Pat. No. 4,842,642, Bowman discusses the use of iron blast furnace slag to flux other components for synthetic ladle slag, especially lime and dolomitic lime, thereby reducing the wear rate of the ladle refractories. This technique is said to hasten the dissolution of MgO into the slag, thus decreasing the duration of non-equilibrium between the MgO in the refractory ladle lining and the MgO dissolved in the slag so that refractory lining wear rate is decreased.
As noted above, attempts are usually made to eliminate primary steelmaking slag from ladle refining processes, either by retention of the primary slag in the primary steelmaking furnace or by raking from the surface of the melt in the ladle, or both. The degree of success of these processes is variable and unpredictable. Some primary slag usually remains on the steel in the ladle when secondary refining is commenced. It is known to add reductants such as ferrosilicon or aluminum to the slag or to the ladle during filling to reduce the iron oxide and manganese oxide carried over in the steelmaking primary slag. However, if added in excess, these reductants dissolve in the steel, making the steel chemistry variable and difficult to predict. In addition the reaction products associated with these reductants may be acidic with respect to ladle refractory linings, and therefore increase the solubility of lining material in the slag thereby increasing the wear rate of the lining. Carbon has been used as such a reductant but the efficiency is low and unpredictable due to the low density of the carbon additives with respect to the slag and reactivity of the carbon with the air above the ladle. Carbon is also soluble in steel, and may be picked up by the steel during the process, thereby altering the steel chemistry which, in many cases, is unacceptable.
The technique of slag foaming has been successfully used in the electric arc steel melting (primary) furnace for the purpose of increasing slag volume, stabilizing the arcs, and lengthening the service life of refractories. The steel furnace is an oxidizing environment (one of the functions is to oxidize carbon out of the steel), so slag foaming is accomplished by injecting particulate carbon and lime into the furnace, along with oxygen blown from a lance, or with iron oxide materials. The carbon combines with the oxygen source to form CO gas and causes foaming of the slag. The injected lime cools the slag to stabilize the foam. This practice has been suggested for use in the ladle, but is not appropriate because of the need to inject oxygen or iron oxide, both undesirable in the ladle steel refining process. Foaming agents that absorb heat when releasing gases are traditionally not used in ladle refining processes.
The use of a reductant for reducing iron oxide in ladle slag is also known, but the materials used thus far have disadvantages. Aluminum is expensive, increases alumina content of the slag, and is an alloying element in the steel, so variability of its concentration is undesirable. Ferrosilicon also results in the increase of slag acidity, and silicon is also an alloying element which has specified composition ranges in steel.